Strange SN75176 failure

[arbj2]

Hi,

I have three PICs connected via RS485 transciever SN75176 to each other. The bus lines are 10 meters long. The lines are A,B and ground.

All the PICs are powered from the same power source an SMPS unit. The system worked perfectly well until one of the units stopped functioning to due a problem in the +5V converter.

Now I find that the SN75176 in one of the other two working units have also failed.

Could anyone explain why this happened ? A power spike in the bus lines could be responsible for this, but how come the other transcievers have not failed ?

I have been using the standard RS 485 scheme for many years without any failures, the bus lines are pulled up/down using 470E resistances, the ground lines are terminated to ground through 120E in each of the units, the chip has 0.1 microF decoupling capacitor.

I know the SN75176 is quite a robust IC, shorting the bus lines also does not damage it as it has a over-temperature shutdown feature to prevent this.

Please explain...


thanks
a

 

[FvM]

Bus voltage beyond 75176 maximum ratings of -10..+15V can damage the device. For a higher degree of surge protection, connect TVS diodes against ground to the 75176 bus terminals.

 

[arbj2]

thanks FvM for the help.

So the connections are...

Cathode of diode 1 to pin 6 and anode to ground, cathode of diode 2 to pin 7 and anode to ground...

Is it correct ??

thanks
a

 

[FvM]

Yes. I'm mostly using unipolar 6.8V TVS diodes.